As a conventional injection head of an injection molding machine, the prior art includes an injection head disclosed in Japanese Patent Application No. 210709/89 which was filed by the same applicant as the present invention and which will be explained below. As shown in FIG. 7, an outside torpedo 2' is arranged behind a nozzle tip 1, in the main body of the injection head 7'. A valve housing 6' arranged in the main body of the injection head 7' is joined to the rear portion of the outside torpedo 2'. An outside passage 8' is formed between the inner peripheral surface of the main body of the injection head 7' and the outer peripheral surface of the valve housing 6'. A center shaft 5' which is reciprocated by a reciprocation drive 17' comprising a hydraulic cylinder, and a piston, and the like is inserted into the valve housing 6' and an inside passage 9' is formed between the valve housing 6' and the center shaft 5'.
In the nozzle tip 1', a conical hole which enlarges to the rear from the nozzle hole 1a', an inner sealing portion 1b' consisting of a large hole having the same diameter in the axial direction as the rear of the conical hole, and a conical hole which enlarges to the rear from the inner sealing portion 1b' are formed. In addition, an inner hole of the outside torpedo 2' is made up of a large diameter portion 2b' and a small diameter portion 2c' which are formed sequentially from the front, the outside torpedo also having a plurality of through holes 2a' formed in the axial direction. An intermediate large diameter portion 3c' of the inside torpedo 3' which is fixed to the center shaft 5' is slidably guided to the large diameter portion 2b' of the outside torpedo 2, In the inside torpedo 3', tip-sealing surface 3a' projects forward from the intermediate large diameter portion 3c' in which a plurality of through holes 3d' are formed in the axial direction and a sealing portion 3b' projects backward from the intermediate large diameter portion 3c'.
A first injection unit 15' for forming skin layers communicates with the outside passage 8' of the injection head. A second injection unit 16' for forming core layers communicates with the inside passage 9' of the injection head. As shown in FIG. 8, the inside torpedo 3' moves forward or backward, thereby opening or closing the outside passage 8' and inside passage 9'. By injecting different kinds of fused resins from the first and second injection units 15' and 16' a sandwich molded article can be formed by opening and closing the passages 8' and 9'.
On the other hand, the prior art also includes injection heads which are coupled to two injection units of a double-head injection molding apparatus, such as injection heads (A) and (B) disclosed in the following Official Gazettes and the like.
(A) An injection head in which an inside nozzle is concentrically arranged inside an outside nozzle and is slidable in the axial direction and rotatable by a rotating means, a resin passage is opened or closed by the rotation of the inside nozzle, and an operating rod to open or close the nozzle hole of the inside nozzle is arranged in the inside nozzle (Japanese Patent Laid-open No. 220341/84).
(B) An injection head in which an annular insert is concentrically arranged in the main body of the injection head and is arranged slidably in the axial direction, a needle having a connecting passage is rotatably arranged in the annular insert, the connecting passage of which is opened or closed by rotating the needle, thereby enabling a gas to by supplied (Japanese Patent Laid-open No. 94805/88).
In the injection head disclosed in No. 210709/89 mentioned above, an inside torpedo has an intermediate large diameter portion. Therefore, as shown in FIG. 8, when the inside torpedo is moved backward at time t.sub.2, only the outside passage is opened, and a resin for a skin layer can be injected. At this time, the volume of the gap portion of the inner hole of the outside torpedo is largely decreased by the intermediate large diameter portion of the inside torpedo, causing the pressure of the gap portion to rise. Therefore, as a result of the backward movement of the inside torpedo, the resins for the core layer filling a plurality of through holes in the inside torpedo during injection molding immediately prior to the backward motion are not held by the inside torpedo but are extruded forward and are left at a position forward of the inside torpedo, that is, in the front edge portion of the outside passage. Thus, a disadvantage of this arrangement is that the resins for the core layer are mixed into the resins for the skin layer which are supplied from the outside passage to the portion forward of the inside torpedo and a defective portion in which these different resins are mixed consequently occurs in the molded article.
To solve the above problem, a large quantity of resin for the skin layer is allowed to pass from the first injection unit, before the normal injection is started to purge the material in which the resin for the core layer left in the front edge portion of the outside passage is mixed with the resin for the skin layer. The molding work(process) is therefore complicated and requires additional time and labor and material costs rise. On the . other hand, even in cases where the inside torpedo is moved backward at time t.sub.7, and only the outside passage is opened and a small quantity of resin for the skin layer is injected, in a manner similar to the above, the resins for the core layer which fill the plurality of through holes of the inside torpedo are not held by the inside torpedo but are extruded forward and are left in the front edge portion of the outside passage. The problem in this case is that the resin for the core layer is mixed into the resin for the skin layer which is supplied from the outside passage to the portion forward of the inside torpedo and consequently, a defective portion in which different materials are mixed still occurs in the molded article.
On the other hand, since the inner sealing portion of the nozzle tip has a large hole which corresponds to the intermediate large diameter portion, upon completion of the injection of the resin for the core layer at time t.sub.8 ', the quantity of resin for the core layer remaining in the large hole increases. Therefore, there remains the problem that even by draining a small amount of resin for the skin layer just after the backward motion, the remaining resin for the core layer cannot be completely discharged and it is therefore injected into the die mixed with the resin for the skin layer and a defective portion occurs in the molded article.
In the above techniques (A) and (B), the opening/closing mechanisms of the nozzle holes are complicated and since a plurality of driving apparatuses are provided, the controls for opening/closing the nozzle are also complicated. Further, these heads have been developed for sandwich molding and, as a result, single molding from a single head is impossible. Also, the opening or closing operation of the nozzle hole of the outside nozzle is executed by sliding the inside cylinder (or annular insert). Thus, there are problems such as cobwebbing of the fused resin from the nozzle hole which occurs with every shot. It is also difficult to make the injection head in a small diameter due to the limitation of the strength of the nozzle structure.